1. Field of the Invention
The present invention relates to an image pickup apparatus and more particularly to an image pickup apparatus for receiving light including image information and producing an electrical signal corresponding to the intensity of the light.
2. Description of the Related Art
As shown in FIG. 8, a conventional image pickup apparatus 50 for receiving light including image information and producing electrical signals corresponding to the light is provided in its front portion with a lens system 51. A charge coupled device (hereinafter abbreviated to "CCD") 53, which is formed of solid-state image pickup elements, is disposed at the focal point of the lens system 51 on which the image of an object 52 is focused.
If the CCD 53 is a linear image sensor of a rectangular shape, as shown in FIG. 11, the CCD 53 is provided with a linear arrangement of image pickup elements 56 having a width w on the order of 10 .mu.m that are linearly arranged parallel to the longer axis of the rectangular shape facing toward the lens system 51. A filter unit 54 is disposed between the lens system 51 and the CCD 53 on an optical path 52a. As shown in FIG. 9, the filter unit 54 comprises a circular filter holding disk 55 holding a red filter 55R, a green filter 55G, and a blue filter 55B arranged at equal angular intervals, a shaft 54C joined to the central portion of the filter holding disk 55, and a step motor 57 coupled with the shaft 54C (FIG. 8). The step motor 57 rotates the shaft 54C to locate the filters 55R, 55G and 55B selectively on the optical path 52a.
As shown in FIG. 8, the CCD 53 is electrically connected to an analog-to-digital converter (hereinafter the analog-to-digital converter is referred as "A/D converter") 58, which converts analog signals output by the CCD 53 into corresponding digital signals. A storage device 59, which is electrically connected to the A/D converter 58, stores digital signals output by the A/D converter 58. A control circuit 60 is electrically connected to the CCD 53, the A/D converter 58, the step motor 57, and the storage device 59 in order to control their operation as described below.
The control circuit 60 drives the step motor 57 to rotate the shaft 54C so as to locate the red filter 55R on the optical path 52a to focus the red image component of the object on the CCD 53. The control circuit 60 controls the CCD 53 so that the CCD 53 outputs a signal representing the quantity of light received, i.e., the integral of the luminance on the CCD 53 in the fixed time. The analog signal output by the CCD 53 is converted into a corresponding digital signal by the A/D converter 58 and the digital signal is stored in the storage device 59. Thus, the red image data of the object is read by the image pickup apparatus 50.
The step motor 57 is driven for rotation of the shaft 54C through an angle of 120.degree. over a fixed time as controlled by the control circuit 60 so that the red filter 55R, the green filter 55G and the blue filter 55B are respectively located on the optical path 52a and held on the optical path 52a for a fixed time. Hereinafter, the foregoing procedure will be referred to as an image pickup procedure.
Referring to FIG. 12, if a time T required for turning the filter holding disk 55 through an angle of 120.degree. coincides with a time T in which the CCD 53 is charged, namely, an integral time (hereinafter the time will often be referred to as read time), red, green and blue image data can be acquired while the filter holding disk 55 is turned one full turn, namely, through an angle of 360.degree.. The angular phase of the filter holding disk 55 is measured to the right on the horizontal axis, and the angular phase of the filter holding disk 55 is 0.degree. when the boundary line L between the red filter 55R and the blue filter 55B (FIG. 9) is in alignment with the CCD 53. Since the filter holding disk 55 is rotated at a fixed rotating speed, the horizontal axis indicating the angular phase may be regarded as a time-base axis.
The green and blue image data of the object 52 are read by the same procedure as that for reading the red image data, and digital signals corresponding to the green and blue image data are stored in the storage device 59 as digital green and blue image data. The digital red, green and blue image data stored in the storage device 59 is sent through an interface IF to an external device (not shown), which includes devices such as a microprocessor.
The image pickup apparatus 50 as described has a disadvantage in that the apparatus 50 requires a comparatively long time for reading the red, green and blue image data because the filters 55R, 55G and 55B are turned through a fixed angle over a fixed, long period of time regardless of the illumination level of environment and, in particular, in the case of high illumination.
If the image pickup apparatus 50 is used for reading a black-and-white image, the filter holding disk 54 is held stationary with one of the filters 55R, 55B, and 55G. For example, the green filter 55G is located in front of the CCD 53 on the optical path 52A to focus a green image on the CCD 53. Thus, the white image data is substituted by the green image data. Accordingly, the gradations between black and white images of the object cannot be discriminated clearly from each other because of the single color filter effect.
Another conventional image pickup apparatus, as shown in FIG. 10, employs a CCD 61 having a plurality of image pickup elements 61A covered respectively with red filters 62R, green filters 62G and blue filters 62B. This image pickup apparatus, however, has a disadvantage in that the resolution thereof is 1/3 that of the image pickup apparatus 50 because 1/3 of the image pickup elements 61A of the CCD 61 are assigned to each of the red, green and blue images, respectively.